Send a copy if radio is checked

// Send copy, if radio checked – Based on http://www.motomax.de/wordpress/contact-form-7-send-copy-to-sender-with-checkbox-in-3-9-update/
function check_mail_send_contactform($cf7) {
    //get CF7's mail and posted_data objects
    $submission = WPCF7_Submission::get_instance();
    if ( $submission ) {
        $posted_data = $submission->get_posted_data();
    }
    $mail = $cf7->prop( 'mail' );

    if(isset($posted_data['checkbox'][0])) { //if Checkbox checked
        $mail2 = $cf7->prop( 'mail_2' ); //get CF7's mail_2 object
        //now set sender's address to mail2's recipient
        $mail2['recipient'] = $posted_data['your-email'];
        $cf7->set_properties( array( 'mail_2' => $mail2 ) );
    }
    return $cf7;
}
add_action('wpcf7_before_send_mail','check_mail_send_contactform');

FYI: This still works with latest Contact Form 7 implementation (5.3) and the latest Wordpress installation (5.5.3).

Note that 'checkbox' reflects the name in the contact form (the second string), e.g.
[checkbox checkbox use_label_element "Send a copy of my email e-mail"]

I was just lazy and decided to call the checkbox field 'checkbox' whereas I can see jose-serrao used the name 'contact_sendmail'.

This did it for me too if(isset($posted_data['contact_sendmail'][0]))